In the grounds of one of the city's oldest Christian churches, Girgis Gabriel Girgis is tattooing a baby girl.
She is very young, only about three years old, and branding the blue cross onto the girl's inside wrist brings a piercing shrill scream.
But for these parents, this is a proud moment. The tattoo symbolises community and identity.
Others queue patiently as Girgis wipes away the dye to reveal a tiny Coptic cross.
They all shout "Allah!", which is the Arabic for God whether you're Christian or Muslim.
There are plenty more who want to be inscribed indelibly as Coptic Christians.
"The tattoo was once used to identify Christian orphans whose parents had been killed in war," said Girgis. "So they wouldn't be brought up as Muslims!"
Ayman Raafat Zaki, 22, also bears a cross.
He has been a member of St Michael's church in Cairo for nine years and he is now an altar boy.
Every Sunday, dressed in his white robes, he helps lead a large Christian congregation.
He chants readings from the Bible, as the young boys circle the church, spreading thick plumes of fragrant incense.
And yet Ayman's overt spirituality - and his tattoo - are not enough to convince the state he is a Christian.
Ayman's father converted to Islam so he could divorce his wife when Ayman was just five months old.
Ayman's mother took her only child and fled the family's village for Cairo.
In Islam, the father determines the religion of his children.
And now - even as an adult - Ayman is denied by the state the Christian identity card he craves.
"Since the age of 16, I have been living an anonymous life," he said.
"In the eyes of the I state, I don't exist. They are trying to force me to become a Muslim by accepting a Muslim identity card. But it was my father's decision to convert. Not mine."
"I'd rather die than accept a Muslim identity card. It is plainly obvious to anyone here I am a practising Christian," he says.
Christians in Egypt comprise about 10% of the country's 80 million people.
But in a predominantly Islamic society, the Copts say they are are being increasingly marginalised.
Forced conversion claims
Identity cards carrying details of a person's religion are required by law in Egypt for employment, education, and access to any public services.
International rights groups say they are also used to discriminate in areas such as employment.
There are other cases involving claims of violent, forced conversion to Islam.
Nahla, whose identity we have protected, says she left home to escape her abusive family.
She moved in with her sister, who had converted to Islam in order to marry a Muslim man.
Within a month of moving in, Nahla's brother-in-law announced he had found her a Muslim husband and pressured her to convert.
After she refused to submit, her brother-in-law reported her to the police and they took her to a police station where she was beaten, she alleges.
Nahla eventually ran away from her Muslim husband, and has now remarried a Christian.
Her children are Christian, she is a regular at her Coptic church, yet she is refused a identity card that says she is Christian by the state.
"You need a card for everything in Egypt, even to be buried," she says.
"Where will they put me when I die? I don't want to be put in a Muslim grave."
In this country, the allegations of forced conversion are hugely divisive, even explosive.
In villages where Christians and Muslims live together, there have been riots over the issue.
Last month, a new report by Christian Solidarity International and the Coptic Foundation for Human Rights documented 25 cases of alleged forced conversion - and criticised the government for ignoring the cases.
But the report has many detractors who argue that it fails to grasp the realities on the ground.
Youssef Sidhoum, the editor of a well-respected Christian newspaper, says the allegations are always difficult to prove.
Often, he says, they are love stories that have gone wrong.
Very often they are not kidnapping or forced conversions, but relationships between Christian girls and Muslim boys.
Sometimes it is their parents who say they have been kidnapped in order to hide their shame, when in fact the girl has married a Muslim of her own choice.
"They tend to exaggerate the cases," he said.
"We have investigated lots of cases, again and again. This is an important issue to us and we go wherever the cases are.
"But I don't recall since 1997 more than three definite cases where we had clear evidence that there was kidnap and forced conversion."
But despite the complexities of alleged forced conversion cases, he and other Christians believe anyone wanting to change their official identity back from Muslim to Christian should be able to do so freely.
There are cases where the Egyptian government plays a direct hand, forcing people to retain a Muslim identity against their wishes.
Lawyer Peter Ramses Al Nagar says he is now representing 3,200 Christians who are forced to live under a Muslim identity.
"The law says when a person becomes 16 years old, when they must get an identity card, he or she has the right to take papers from the church to prove they are Christian.
"But there are people who have taken these papers to the interior ministry and they have been told they have two choices.
"Either they take a Muslim identity card or they live without an identity card, which is a major, major problem."
There have been cases where people in the same position as Ayman and Nahla have successfully challenged the state in court, but very often the interior ministry simply ignores the ruling.
And more recently, courts have tended to take the side of government lawyers, who argue that a return to Christianity is apostasy.
Under some interpretations of Islamic law, apostasy - conversion from Islam - is punishable by death.
According to Human Rights Watch, such conversions are not banned under Egyptian law, but the courts have viewed rulings that would be perceived as sanctioning them as potential offences against public order.
The BBC did ask the interior ministry for a response to our investigation, but after three weeks we have had no answer.
No government representative has been put forward for comment.